As an engine-cooling fan for an internal combustion engine of an automobile or a similar engine, there has been known a fan in which a cylindrical hub and a plurality of fan blades provided radially on the outer peripheral wall of the hub are integrally injection-molded using a synthetic resin as a raw material.
Incidentally, an engine-cooling fan is used under the influence of violent vibrations of an engine and moreover, at a high revolutionary speed. Thus, the engine-cooling fan is repeatedly subjected to influence such as resonance phenomena due to vibrations of the engine or reaction forces of wind pressures. However, conventional engine-cooling plastic fans often developed cracked damages due to fatigue because the rigidity of the connecting part between each fan blade and its associated hub was not sufficient.
It may hence be contemplated, for example, of increasing the thickness of a heel portion of each fan blade or the curvature of radius of the heel portion of each fan blade so as to increase the rigidity of a connecting part between the heel portion and the hub. An approach of this sort is however accompanied by a drawback that where the hub and fan blades are injection-molded integrally with a synthetic resin, one or more cavities may be formed within the thus-thickened portion or an irregular deformation may occur due to molding sink near the thickened portion and the fatigue strength may hence be reduced on the contrary.